It's a Nurses graduation pin (Lamp of Knowledge). S.U. could be Shenandoah University, Salisbury University, both have nursing programs. Or.......just about any nursing program with those initials.
Can you read the words on the back (or take a close up)? Also letting folks know where you are (England, right?) makes it easier to help you.
The lamp of knowledge is used outside of nursing (as well) as a symbol of academic achievement.
Agreeing with Nhbenz... that "Aladdin-style" oil-lamp, with a flame coming out to show it is lit, giving light, represents "the Lamp Of Knowledge." That specific symbol has been frequently used in Academic/Scholarly emblems for centuries, particularly by schools & universities, not just Nursing.
Well-focused closeup photos of the maker's-mark on your pin's back may give us information which will help us identify it.
The specific form of pin-clasp on its back dates from the 1920s onward to the present.
So, as JonG indicated, the letters "S.U" on your pin are highly likely to stand for a University whose name starts with an S.
"it was just the scripture union. it was associated in schools and at the Royal Infirmary Edinburgh there was a nurses branch so members would wear it. not directly attributed to any particular school of nursing just for those who had a religeous interest."